One of the most significant developments to take place in communications technology has been the increasing availability of multimedia services that combine and coordinate video with voice and text applications. An important aspect in delivering multimedia services over communications networks is the translation or “transcoding” of media streams such as audio and/or video into formats supported not only by the networks that carry the media streams, but also the endpoint devices that receive the media streams, such as traditional personal computers (PCs), mobile PCs, personal digital assistants (PDAs), video-enabled mobile phones, mobile televisions (TVs), third generation (3G) phone sets, or any other suitable multimedia systems or devices.
Rate control is a vital function in video transcoding systems due to its impact on video quality. A conventional video transcoding system typically includes a buffer that receives an input video stream compressed according to a first coding format, a video decoder that decodes the input stream in the first coding format, and a video encoder that encodes the decoded stream according to a second coding format, thereby producing an output video stream in the second coding format. Although video encoders typically encode video as a variable bitrate stream, many multimedia applications and networks require a constant bitrate stream to assure good quality video. For this reason, rate control algorithms have traditionally been implemented within such video transcoding systems to control the bitrate of the video encoder output, thereby adapting the video transcoding system to the network environment in which it is employed.
Various types of rate control algorithms have been developed for controlling the output bitrate of video encoders. For example, model based adaptive rate control algorithms that can adapt to the dynamic characteristics of video streams have been employed in standalone video encoders to produce encoded video sequences of good quality. However, using such conventional model based adaptive rate control algorithms in video transcoding systems can sometimes have a detrimental effect on video quality because the characteristics of the decoded video stream received as input by the video encoder within the video transcoding system are generally different from the characteristics of the input video frames received by the standalone video encoder. Such differences in the characteristics of the decoded video stream generated within the video transcoding system may cause conventional rate control algorithms to make inefficient allocations of bits across the video stream. For example, in the video transcoding system, too many bits might be allocated to some video frames, leaving too few bits for encoding the remaining video frames, thereby causing “blocky” or “blurry” video in those frames that were allocated an insufficient number of bits. Moreover, those frames that were encoded using too many bits may require an inordinate amount of time to be transmitted through a constant bitrate network, resulting in, for example, poor lip synchronization of the video and audio signals at the endpoint device.
It would therefore be desirable to have an improved rate control algorithm for use in a video transcoding system that avoids one or more of the drawbacks of conventional rate control algorithms.